Combining iterative metal artifact reduction and virtual monoenergetic images severely reduces hip prosthesis-associated artifacts in photon-counting detector CT

Aim of this study was to assess the impact of virtual monoenergetic images (VMI) in combination and comparison with iterative metal artifact reduction (IMAR) on hip prosthesis-associated artifacts in photon-counting detector CT (PCD-CT). Retrospectively, 33 scans with hip prosthesis-associated artifacts acquired during clinical routine on a PCD-CT between 08/2022 and 09/2022 were analyzed. VMI were reconstructed for 100–190 keV with and without IMAR, and compared to polychromatic images. Qualitatively, artifact extent and assessment of adjacent soft tissue were rated by two radiologists using 5-point Likert items. Quantitative assessment was performed measuring attenuation and standard deviation in most pronounced hypodense and hyperdense artifacts, artifact-impaired bone, muscle, vessels, bladder and artifact-free corresponding tissue. To quantify artifacts, an adjusted attenuation was calculated as the difference between artifact-impaired tissue and corresponding tissue without artifacts. Qualitative assessment improved for all investigated image reconstructions compared to polychromatic images (PI). VMI100keV in combination with IMAR achieved best results (e.g. diagnostic quality of the bladder: median PI: 1.5 (range 1–4); VMI100keV+IMAR: 5 (3–5); p < 0.0001). In quantitative assessment VMI100keV with IMAR provided best artifact reduction with an adjusted attenuation closest to 0 (e.g. bone: PI: 302.78; VMI100keV+IMAR: 51.18; p < 0.0001). The combination of VMI and IMAR significantly reduces hip prosthesis-associated artifacts in PCD-CT and improves the diagnostic quality of surrounding tissue.

Imaging protocol. A weight-adapted volume of iodine based contrast agent (Accupaque 300 mg/ml, GE Healthcare Buchler GmbH & Co. KG, Braunschweig, Germany) was applied intravenously with a flow rate of 3 ml/s followed by a bolus of 40 ml of physiologic saline solution.
Scan parameters were a tube voltage of 120 kVp with activated automatic tube current modulation, a pitch of 0.8 and a gantry rotation time of 0.5 s. Detector collimation was 144 × 0.4 mm. Reconstruction parameters were 1 mm slice thickness with an increment of 0.7 mm. Scans were performed in a head-first supine position. A regular kernel (QR40; Siemens Healthcare GmbH, Erlangen, Germany) and Quantum Iterative Reconstruction (QIR Level 3; Siemens Healthcare GmbH, Erlangen, Germany) was used for image reconstruction.
VMI were reconstructed in axial view for 100-190 keV in an interval of 10 keV using dedicated software (syngo.via VB 60, Monoenergetic Plus; Siemens Healthcare GmbH, Erlangen, Germany). Iterative metal artifact reduction optimized for hip prosthesis (iMAR, Siemens Healthcare GmbH, Erlangen, Germany) is commercially available and was used as supplied by the vendor.
Quantitative image analysis. Quantitative assessment of polychromatic and virtual monochromatic images was performed by region of interest (ROI) based attenuation analysis. ROIs were placed in the most pronounced hypodense and hyperdense artifacts using a conventional clinical DICOM viewer (Deep Unity R20 XX; Dedalus HealthCare GmbH, Bonn, Germany). Furthermore, values and standard deviation of X-ray attenuation was evaluated in muscle tissue, bone and bladder tissue with and without presence of artifacts. Hereby, differences in attenuation and gradient of attenuation of tissue were measured and an adjusted attenuation and image noise assessed as proposed previously 15 . Adjusted attenuation was assessed to differentiate artifact reduction from regular changes in Hounsfield units (HU) for differing VMI energy levels. Adjusted attenuation was calculated as the difference of tissue impaired by artifacts and without artifact impairment. Image noise is higher in images with presence of artifacts and VMI of high keV, therefore we calculated an adjusted image noise using the difference of standard deviation between muscle tissue in HU values in areas with and without artifacts. Adjusted attenuation of 0 indicates optimal artifact reduction, values above 0 indicate insufficient artifact reduction and values below 0 an overcorrection of the artifact correction. All measurements were performed for polychromatic and virtual monochromatic images from 100 to 190 keV in steps of 10 keV.
Qualitative image analysis. Two radiologists with two (YCL) and eleven (DK) years of experience in abdominopelvic CT evaluated the CT images independently regarding artifact extent of hyperdense and hypodense artifacts as well as assessment of surrounding tissue using a five-point Likert grading scale. Rating of artifacts was defined as follows: (1) excessive artifacts; (2) pronounced artifacts; (3) moderate artifacts; (4) minor artifacts; and (5) artifacts are absent. For assessment of diagnostic quality of bone, muscle, vessels, bladder and soft tissue, the following Likert scale was used: (1) highly restricted diagnostic interpretability; (2) restricted diagnostic interpretability; (3) moderate diagnostic interpretability; (4) minor restrictions on diagnostic interpretability; and (5) unrestricted diagnostic interpretability. Additionally, both readers had to select the reconstruction with the best diagnostic quality for each patient. Polychromatic images as well as VMI with 100 keV, 130 keV, 160 keV and 190 keV were rated. Statistical analysis. All statistical analyses were conducted using IBM SPSS Version 27 (IBM Corp., Armonk, NY, USA). Graphs were carried out using the software GraphPad PRISM Version 6.02 (GraphPad Software, San Diego, CA, USA). Quantitative results are stated as mean and standard deviation. Wilcoxon signedrank test was used for statistical analysis of quantitative image parameters. Qualitative results are expressed as median with interquartile range (IQR). Interrater reliability was assessed using the intraclass correlation coefficient (ICC). ICC estimates and their 95% confident intervals (CI) were calculated based on a mean-rating (k = 2), consistency, two-way mixed-effects model. p values below 0.05 were considered significant. Quantitative image analysis. Results of quantitative analysis are summarized in Supplemental Information 1 and Fig. 2; Fig. 3 shows typical reconstructions of a pelvic CT with unilateral hip replacement and Fig. 4 shows representative images of a pelvic CT with bilateral hip replacement using VMI and IMAR. VMI 100keV+IMAR showed the best adjusted attenuation overall compared to PI with the closest value to 0 (  . Hyperdense artifacts were subjectively reduced, scoring a 5 (3-5, p < 0.05) for all reconstructions combining VMI and IMAR. For the PI the score was 1 (1-3) and VMI 190keV scored 2 (1-4; p < 0.05). For hypodense artifacts, PI scored 1 (1-2), VMI 190keV scored 2 (1-4; p < 0.05) and combination of VMI 100keV and IMAR scored 5 (3-5; p < 0.05). In many cases especially hypodense artifacts adjacent to the prosthesis increased with higher keV VMI (Fig. 5).
Rating results for all criteria by each rater are shown in Fig. 6

Discussion
This study evaluates PCD-CT artifact reduction capabilities regarding hip implants in clinical imaging. The main findings are that VMI 100keV in combination with IMAR deliver best results regarding artifact-reduction and diagnostic quality. Every combination of VMI and IMAR led to artifact reduction compared to conventional polychromatic images, with low keV VMI images demonstrating best contrast, especially for vascular delineation in scans with iodine contrast. As hip arthroplasty and associated revision surgery procedures are projected to increase, there is a growing need for precise, fast and artifact-free imaging including dedicated scans for evaluation of hip arthroplastyassociated pathologies 16 . PCD-CT is an emerging technology with numerous potential advantages compared with EID-CT based on the direct detection and weighting of every photon 17,18 . Employing energy-thresholds allows for elimination of image background noise; virtual higher-energy image reconstructions are less affected by artifacts typically originating from lower energy photons 19 . High photon sensitivity is a further inert advantage of PCD-CT; diagnostic image quality is achievable with lower tube current, than necessary with EID CT.
Several studies have investigated the effects of VMI and IMAR for reduction of artifacts in dual energy EID-CT 9,20-26 . A recent study investigating split-filter EID-CT with images reconstructed at 120 kVp-equivalent, reported that PI images in combination with IMAR leads to best artifact reduction 15 . In contrast, most previous EID-CT studies reported best artifact reduction for high-energy VMI in combination with IMAR 23-26 .

Figure 2.
Mean adjusted attenuation and standard deviation for bladder, bone, muscle and overall analyzed tissue. Adjusted attenuation was calculated as difference between artifact-impaired tissue and corresponding tissue without artifacts. Values closest to zero show most favorable artifact reduction. VMI 100keV+IMAR showed the best adjusted attenuation overall compared to PI with the closest value to 0 (overall PI: 634.66; VMI 100keV+IMAR : 102.81; p < 0.0001). www.nature.com/scientificreports/ Even though this study did not compare PCD-CT to EID-CT, images achieved were of high diagnostic quality and therefore offer a feasible solution for patients with hip prosthesis suspected of pathologies masked by implant-associated artifacts (Fig. 7).

Scientific Reports
As signal acquisition of PCD-CT differs from dual energy EID-CT, with PCD-CT technology enabling imaging with lower noise and higher spatial resolution, lower energy PCD-CT VMI images are expected to have less noise and higher image quality. In fact, in PCD-CT artifact extent remains relatively stable between 120 and 190 keV, with 190 keV images showing severely reduced contrast; Images reconstructed at 100 keV combined with IMAR resulted in best artifact reduction and showed superior contrast compared to high-keV images. www.nature.com/scientificreports/ A recent phantom study on PCD-CT reported best artifact reduction for 100 keV at scans performed with 120 kVp, which is in line with the results of this study 27 . As recently reported for EID-CT, in PCD-CT IMAR is superior to VMI for the reduction of extensive artifacts but may lead to distortion and blurring. VMI effectively reduces minor artifacts without distorting images 15,23 .
Laukamp et al. reported optimal keV reconstructions for artifact reduction in EID-CT depending on the affected tissue; 200 keV for bone, 140 keV for soft tissue 9 . This could not be confirmed for PCD-CT in the current study. Quantitatively, there were minor, insignificant deviations of adjusted attenuation for muscle and bladder www.nature.com/scientificreports/ towards VMI with higher keV. Nevertheless, VMI 100 keV in combination with IMAR led to optimal results for both quantitative and qualitative assessment of artifact reduction and image quality. As reported previously 15,23,28 , combining IMAR and high keV VMI led to the appearance of new hypodense artifacts adjacent to implants, suggesting a local overcorrection of artifacts. These new artifacts intensify with higher keV VMI and thereby limit diagnostic assessment of surrounding tissue.
There are limitations to the study. Study design was monocentric and therefore only a small cohort of patients was retrospectively included. No distinction was made between unilateral and bilateral hip prostheses. Furthermore, prosthesis composition was not taken into account. As stated before, based on the material and shape Table 1. Mean adjusted noise and adjusted attenuation values and standard deviation within defined regions of interest for polyenergetic reconstruction (PI) and virtual monoenergetic images (VMI) with and without iterative metal artifact reduction (IMAR). Adjusted attenuation closest to 0 shows most favorable artifact reduction. Adjusted noise was calculated for expected lower image noise in high keV and addresses noise without presence of artifacts.

Adjusted noise
Adjusted attenuation (muscle) Adjusted attenuation (bone) Adjusted attenuation (bladder) www.nature.com/scientificreports/ www.nature.com/scientificreports/ varying artifact extent can be assumed 3,29 . Further multi-center evaluation on larger cohorts, especially focusing on patients needing a hip prosthesis revision should be part of further subsequent studies. Overall, the current results show that the combination of IMAR and VMI are the most potent tool for reduction of even extensive hip prosthesis associated artifacts in PCD-CT. The combination of IMAR and VMI improves diagnostic readability and thus may enable the detection of pathologies that would otherwise be concealed by artifacts in conventional CT images.

Data availability
The anonymized datasets generated during and analyzed during the current study are available from the corresponding author on reasonable request. Due to local privacy laws, CT images can not be provided as theoretically there is a risk of identification of personal information in pseudo-anonymized CT datasets.